A demand for transmitting signals while securing electrical insulation between circuits has been obtained in various electronic devices. When a high-voltage circuit and a low-voltage circuit are operated, for example, a malfunction or a failure of the low-voltage circuit is prevented, or a ground loop of circuits of different installation potentials is disconnected. With such a configuration, when the circuits are connected, application of an excessive voltage to one circuit is prevented. A specific example is a case of controlling a motor driving circuit that operates at a high voltage of a few hundred V. When a high voltage to be handled by a motor driving circuit is applied to an input/output unit of a microcomputer that operates at a low voltage, this application can cause a malfunction, and can lead to a failure. To prevent these problems, the low-voltage circuit and the high-voltage circuit are insulated from each other.
So far, as an insulation element that performs communication while achieving insulation, a photocoupler has been mainly used. The photocoupler has a light emitting element and a light receiving element integrated into one package, and the light emitting element and the light receiving element are electrically insulated from each other inside the package. This is a system that coverts an input electrical signal into an optical signal by the light-emitting element, transmits the converted optical signal in an electrically insulated space, detects the transmitted optical signal by the light receiving element, converts the signal into an electrical signal again, and transmits the converted electrical signal. However, the photocoupler has problems in aged deterioration, large power consumption, and the like.
In order to solve these problems, an insulation element called an electromagnetic resonant coupler as described in Patent Literature 1, for example, is known. This is a device that transmits high-frequency signals between circuits located on different planes. In this device, there are formed resonators having a structure that a part of a closed curve line is opened on each plane of different planes, and an input/output line connected to the resonators and being configured to input and output a high-frequency signal to the resonators. The device transmits a high-frequency signal by electromagnetically coupling the resonators formed on both planes. The high-frequency signals mean microwaves or millimeter waves.
In this structure, each resonator is formed in a size of λ/2 of a line length, and the resonators become two antennas formed on what is called the different planes. Therefore, when a gap between the resonators is equal to or smaller than a fixed gap, the resonators are coupled by a near field, and very high efficient transmission becomes possible.